David Bain's sister Laniet was planning to "blow the whistle" on incest with her father and her prostitution the weekend before she and her family were killed, a court has been told.

Laniet Bain had intended to reveal the facts in front of her family, but the High Court has heard that David Bain also wanted to meet the whole family that weekend.

A friend of Laniet, who cannot be named, told the Bain murder retrial yesterday that Laniet was "nervous but happy" when they met in Dunedin's Octagon on Thursday, June 16, 1994 - four days before five of the Bain family were shot dead in their Dunedin home.

The friend said Laniet, 18, told her she had quit being a prostitute and "that she was going home that weekend to blow the whistle on her being a sex worker ... and that she had been having an incestuous relationship with her father".

She agreed with prosecutor Cameron Mander that Laniet also told her that David wanted to meet the whole family that weekend. Laniet did not know why that was.

Bain, 37, is on trial for the murder of his parents and three siblings.

His defence team argue that his father Robin, 58 - fearful of the incest being exposed - shot dead the family before turning the rifle on himself.

Laniet's friend told the court they lived in the same Dunedin boarding house in late 1993. Laniet told her she was working as a prostitute and had been a victim of incest.

She was working in the sex industry because her parents would not sign a form that would allow her to collect the youth benefit, the friend said.

Laniet also told her she had been raped at 10 years old by a family friend when living in Papua New Guinea.

She said she had had a baby as a result, and showed her friend a photograph of a "white-ish" child that had "Laniet's hair".

The witness said she saw Laniet naked once when living together, and noticed she had stretch marks down her stomach.

Previous witnesses have stated that Laniet told them she had given birth to a "black" baby in Papua New Guinea, and had had an abortion.

The friend said Laniet had been blackmailed by a man named Dean Cottle over the knowledge of her work as a prostitute, and forced to have sex with him at least once a week and do "horrible and graphic things".

Laniet was a heavy cannabis user and craved affection, she said.

The friend said she was very surprised to learn that Laniet had lived for a period before the killings with her father at a school house at Taieri Mouth, because she did not like being around her father.

Another witness, Linda Miller, told the court that in 1993 and 1994 she was working in Dunedin massage parlours and got to know Laniet as "another working girl".

Laniet was distressed about an incestuous relationship with her father - "that he was raping her basically", Ms Miller said.

Sean Clarke, a friend of David Bain, told the court via video link from England that in May 1994 Laniet wanted to move out of the school house because she could not stand what her father was doing to her any more.